Two separate relativistic effects push a GPS satellite's clock in opposite directions — and they don't cancel. The leftover drift is engineered out before the satellite ever leaves the ground.
GPS positioning is, at its core, a timing problem: a receiver measures how long a radio signal took to arrive from several satellites and converts that into distance using the speed of light. That means the whole system lives or dies on how precisely satellite clocks stay in sync with clocks on the ground. It turns out ordinary orbital mechanics — the part everyone expects — is nowhere near the whole story. Two independent predictions of relativity, working in opposite directions and at very different magnitudes, change the rate of a satellite clock enough to break the system in minutes if left uncorrected.